In the photolysis of ozone, one of the oxygen-oxygen bonds in the molecule breaks. A specific quantity of energy must be added to break the bond. This is the bond energy.
We can determine the energy necessary to break the O-O bond in ozone with an experiment.
Remember that the energy in light is proportional to its frequency and inversely proportional to the wavelength the light.
E = hc/
Lasers can emit light of a single wavelength, and certain types of lasers can be tuned so that they emit light over a range of wavelengths. By changing the wavelength of laser light through a sample until the radiation causes the O-O bond to break, we can experimentally determine the exact bond energy.
Absorption spectroscopy will tell us when absorption and bond-breaking happens.
When we find the longest wavelength (lowest energy) radiation that can break the bond, we can use it to calculate the bond energy.
For 1 molecule:
E = hc/
c = (3.0 x 108 m/s)(1.0 x 109 nm/m) = 3.0 x 1017 nm/s
h = 6.63 x 10-34 J s = 6.63 x 10-37 kJ s
For 1 mole, multiply the energy required to break the bond in 1 molecule by the number of molecules in a mole:
E = (hc/ molecule)(6.02 x 1023 molecules/mol)
E = (6.63 x 10-37 kJ s)(3.0 x 1017 nm/s)(6.02 x 1023 mol-1)/ E = 1.2 x 105 kJ nm mol-1/
The lowest energy light that can break the bond in ozone has a wavelength of 330 nm.
BE (ozone) = 1.2 x 105 kJ nm mol-1/330 nm = 364 kJ/mol
Other O-O Bonds
We can obtain experimental data on bond energies of other molecules in the same way. Molecular oxygen, O2, is photolyzed by light of 241 nm and has a bond energy of 498 kJ/mol. Hydrogen peroxide, HOOH, has a very weak O-O bond and is photolyzed by light of 845 nm. Its bond energy is only about 142 kJ/mol.
Why do we see such large difference in the strength of oxygen-oxygen bonds in these molecules. Let's look at the Lewis structures.
The bond energy correlates with the bond order.
Bond energies of some gas phase molecules are listed below.
Professor Patricia Shapley, University of Illinois, 2012